Process for conducting equilibriun-limited reactions

ABSTRACT

This invention relates to a process for conducting equilibrium-limited reactions, such as esterification and alcoholysis reactions, that uses two reaction zones. The first reaction zone operates under reaction conditions that retain at least a portion of the product in a liquid phase, and at least a portion of the liquid from the first reaction zone is introduced into a second reaction zone which operates under conditions sufficient to crack heavies, for example, Michael-addition heavies, formed in or introduced into said second reaction zone and to vaporize at least a portion of the product upon production thereof. Such a process allows for removal of product from the reaction system while catalyst desirably remains in the reaction system.

BACKGROUND OF THE INVENTION

This invention pertains to a process for producing a reaction productvia equilibrium-limited reactions, such as esterification andalcoholysis (or transesterification) reactions, wherein the process isconducted using two reactor stages and the desired product or productsof the equilibrium-limited reactions are removed in the vapor phase,preferably from the second stage reactor.

Equilibrium-limited reactions generally involve the reaction of two ormore reactants to produce at least one product and, typically, acoproduct. In order to achieve a greater conversion to the desiredproduct(s), various techniques have been suggested such as removing thecoproduct and/or product from the reaction menstruum to maintain adriving force toward the product.

Equilibrium-limited reactions are often conducted in a single reactorwith product being selectively removed from the reaction menstruum or ina plurality of reactors in which product is separated from the reactionmenstruum in each of the reactor stages. One type of multistage reactorprocess is disclosed in U.S. Pat. Nos. 5,811,574 and 5,900,125, whichdisclose a process and an apparatus for the continuous preparation ofalkyl esters of (meth)acrylic acid by reacting (meth)acrylic acid andmonohydric alkanols of 1 to 8 carbon atoms in the homogeneous, liquid,solvent-free phase at elevated temperatures and in the presence of anacidic esterification catalyst. In the disclosed process, the(meth)acrylic acid, the alkanol and acid catalyst are continuously fedto a reaction zone which consists of a cascade of at least two reactionregions connected in series, and the discharge stream of one reactionregion forms a feed stream of a downstream reaction region. The cascademay have from two to four reaction regions spatially separated from oneanother. These patents disclose an aqueous azeotropic distillationprocess in which the target alkyl acrylate formed in the reaction zoneis separated from the catalyst and starting acid via the top of arectification zone mounted on the reaction zone as a component of atleast one azeotropic mixture consisting of water or water and startingalkanol as further components in addition to the alkyl acrylate. Theresulting distillate is separated into at least one organic phasecontaining the alkyl acrylate and into at least one water-containingaqueous phase. A part of the organic phase containing alkyl acrylate isrecycled via the top of the rectification zone. The remaining organicphase, which contains no starting acid, is fed into downstream equipmentto isolate the target alkyl acrylate from starting alkanol and otherimpurities having lower and higher boiling points than the target ester.It is also disclosed that this process is preferably to be used for thepreparation of n-butyl acrylate because the boiling points of n-butylacrylate and acrylic acid are comparatively close together. It isfurther disclosed that the water content in the water/alkyl acrylate orwater/alkanol/alkyl acrylate azeotropic mixtures increases along withthe increase of the molecular weight of the alkyl acrylate. Higherenergy usage and larger equipment are needed if this azeotropicdistillation process is used for the preparation of higher molecularweight acrylates, such as 2-ethylhexyl acrylate.

U.S. Pat. No. 5,883,288 discloses a process for the continuouspreparation of alkyl esters of (meth)acrylic acid by reacting(meth)acrylic acid and monohydric alkanols of 1 to 8 carbon atoms in thepresence of an acidic esterification catalyst in a reaction zone. Thereaction zone can consist of a cascade of at least two reaction regionsconnected in series, and the discharge stream of one reaction regionforms a feed stream of a downstream reaction region. The cascade mayhave from two to four reaction regions spatially separated from oneanother. A product mixture is discharged from the reaction zone and fedto a rectification unit (I) and separated into at least one productcomprising the alkyl ester of (meth)acrylic acid and one productcomprising the catalyst. The alkyl ester of (meth)acrylic acid productis then fed to a further rectification unit (II) and separated off byrectification.

The disadvantage of such a process is that the acid catalyst is sentwith the reaction mixture from the reaction zone to a rectification unit(I), rather than being allowed to remain in the reaction zone. Thecombination of high acid catalyst concentration and high temperatures inthe lower section and the bottom of the rectification unit (I) lead tohigh rates of corrosion, equipment fouling, and undesired sidereactions, such as decomposition of the acid catalyst.

Accordingly, an improved process for conducting equilibrium-limitedreactions is sought that would minimize and/or eliminate the problemsassociated with conducting the reactions and with recovering unreactedreactants and product.

SUMMARY OF THE INVENTION

The process of this invention relates to conducting anequilibrium-limited reaction in at least two reaction zones, wherein atleast a portion of a liquid phase reaction menstruum containing productproduced in a first reactor zone is supplied to a second reaction zoneand wherein product produced in the first reaction zone is also producedin the second reaction zone and is removed from the second reaction zonein the vapor phase. The temperature and pressure in the second reactionzone are sufficient to crack heavies, for example, Michael-Additionheavies, formed in or introduced into said second reaction zone and tovaporize at least a portion of the product upon production thereof. Sucha process allows for removal of product from the reaction system whilecatalyst desirably remains in the reaction system. The process of thisinvention does not require the use of an aqueous azeotropic distillationcolumn to separate the target ester product from the starting acid.While typically only one product is ultimately sought, the process ofthis invention can simultaneously make two or more products. Forinstance, acrylic acid may be reacted with a mixture of 2-ethylhexanoland butanol to produce the corresponding 2-ethylhexyl and butylacrylates.

For the purposes of the present invention, the term “heavies” meanscompounds having a boiling point higher than that of the target esterproduct. For a process producing more than one ester product, heaviesare compounds having a boiling point higher than the boiling point ofthe highest boiling ester product. Examples of heavies includeoxyesters, such as alkoxy propionates or acryloxy propionates, formedvia the Michael-addition reaction of the target ester product with thestarting alkanol and/or starting acid.

The process of this invention relates in part to conducting anequilibrium-limited reaction of at least one reactant to produce atleast one product, comprising:

a. reacting said at least one reactant in a first reaction zonemaintained under reaction conditions, including temperature andpressure, sufficient to produce said at least one product and sufficientto maintain at least a portion of said at least one reactant and said atleast one product in the liquid phase;

b. withdrawing a liquid fraction containing said at least one reactantand said at least one product from said first reaction zone, andintroducing at least a portion of said withdrawn liquid fraction into asecond reaction zone maintained under reaction conditions, includingtemperature and pressure, sufficient to (i) produce said at least oneproduct, (ii) crack heavies formed in or introduced into said secondreaction zone and (iii) vaporize at least a portion of said at least oneproduct upon production thereof;

c. withdrawing an overhead vapor fraction from said second reactionzone, said overhead vapor fraction comprising said at least one product,and introducing at least a portion of said withdrawn overhead vaporfraction into at least one condensation zone to produce said at leastone product in the liquid phase;

d. withdrawing a liquid fraction from said at least one condensationzone, said liquid fraction comprising said at least one product andwater, and introducing at least a portion of said withdrawn liquidfraction into at least one separation zone to provide by phaseseparation an organic liquid fraction comprising said at least oneproduct and an aqueous liquid fraction comprising water;

e. withdrawing said organic liquid fraction from said at least oneseparation zone, said organic liquid fraction comprising said at leastone product, and introducing at least a portion of said withdrawnorganic liquid fraction into at least one reactant recovery distillationzone to provide an overhead fraction comprising said at least onereactant and a bottoms liquid fraction comprising said at least oneproduct;

f. withdrawing said bottoms liquid fraction from said at least onereactant recovery distillation zone, said bottoms liquid fractioncomprising said at least one product, and introducing at least a portionof said withdrawn bottoms liquid fraction into at least one productrecovery distillation zone to provide an overhead fraction comprisingsaid at least one product and a bottoms liquid fraction comprisingheavies; and

g. recovering said at least one product from said withdrawn overheadfraction. In this embodiment, lower purity feed streams, for example,crude 2-ethylhexanol or crude acrylic acid streams containing highconcentrations of acrylic acid dimer or other Michael-addition heaviesmay be utilized in the process of this invention. In one embodiment,heavy residue-containing streams generated from other processes thatemploy similar equilibrium-limited reactions, for example, integratedequilibrium-limited processes, can be employed as feedstocks. Also, inthis embodiment, the product can be efficiently removed from thereaction zone while the catalyst desirably remains in the reaction zone.

The process of this invention also relates in part to conducting anequilibrium-limited reaction of at least one reactant to produce atleast one product, comprising:

a. reacting said at least one reactant in a first reaction zonemaintained under reaction conditions, including temperature andpressure, sufficient to produce said at least one product and sufficientto maintain at least a portion of said at least one reactant and said atleast one product in the liquid phase;

b. withdrawing a liquid fraction containing said at least one reactantand said at least one product from said first reaction zone, andintroducing at least a portion of said withdrawn liquid fraction into asecond reaction zone maintained under reaction conditions, includingtemperature and pressure, sufficient to (i) produce said at least oneproduct, (ii) crack heavies formed in or introduced into said secondreaction zone and (iii) vaporize at least a portion of said at least oneproduct upon production thereof;

c. withdrawing an overhead vapor fraction from said second reactionzone, said overhead vapor fraction comprising said at least one product,and introducing at least a portion of said withdrawn overhead vaporfraction into at least one condensation zone to produce said at leastone product in the liquid phase, and introducing at least onepolymerization inhibitor into said at least one condensation zone;

d. withdrawing a liquid fraction from said at least one condensationzone, said liquid fraction comprising said at least one product andwater, and introducing at least a portion of said withdrawn liquidfraction into at least one separation zone to provide by phaseseparation an organic liquid fraction comprising said at least oneproduct and an aqueous liquid fraction comprising water;

e. withdrawing said organic liquid fraction from said at least oneseparation zone, said organic liquid fraction comprising said at leastone product, and introducing at least a portion of said withdrawnorganic liquid fraction into at least one reactant recovery distillationzone to provide an overhead fraction comprising said at least onereactant and a bottoms liquid fraction comprising said at least oneproduct, and introducing at least one polymerization inhibitor into saidat least one reactant recovery distillation zone;

f. withdrawing said bottoms liquid fraction from said at least onereactant recovery distillation zone, said bottoms liquid fractioncomprising said at least one product, and introducing at least a portionof said withdrawn bottoms liquid fraction into at least one productrecovery distillation zone to provide an overhead fraction comprisingsaid at least one product and a bottoms liquid fraction comprisingheavies, and introducing at least one polymerization inhibitor into saidat least one product recovery distillation zone;

g. withdrawing from said at least one product recovery distillation zonea bottoms fraction comprising at least one polymerization inhibitor andsupplying at least a portion of the withdrawn bottoms fraction to saidat least one condensation zone, said at least one reactant recoverydistillation zone, said at least one product recovery distillation zoneand a water removal distillation zone, in an amount sufficient tominimize or eliminate polymerization of said at least one reactant andsaid at least one product; and

h. recovering said at least one product from said withdrawn overheadfraction.

In this embodiment, polymerization inhibitors are reused in the processby recycling the withdrawn bottoms fraction containing polymerizationinhibitors from the at least one product recovery distillation zone tothe at least one condensation zone, the at least one reactant recoverydistillation zone, the at least one product recovery distillation zoneand the water removal distillation zone. In addition to being costeffective, the recycling of polymerization inhibitors to the variousdistillation zones controls undesirable polymerization. For example,reactive monomers such as acrylic acid and 2-ethylhexyl acrylate readilyform polymer via free radical polymerization if not well inhibited. Thisprocess is further cost effective in that the recycle stream returnsheavies to the second reaction zone for cracking.

In another embodiment, the above process also comprises generating inthe first reaction zone and, under reaction conditions of the firstreaction zone, a vapor fraction comprising water, at least a portion ofthe one reactant and at least a portion of the one product, withdrawingthe vapor fraction from the first reaction zone and introducing thevapor fraction into at least one water removal distillation zone,withdrawing the overhead vapor fraction from the at least one waterremoval distillation zone, introducing at least a portion of thewithdrawn overhead vapor fraction into at least one condensation zone toproduce a liquid fraction, withdrawing the liquid fraction from the atleast one condensation zone, the liquid fraction comprising water, atleast a portion of the one reactant and at least a portion of the oneproduct, introducing at least a portion of the withdrawn liquid fractioninto at least one separation zone to provide by phase separation anorganic liquid fraction comprising the at least a portion of the atleast one reactant and at least a portion of the at least one productand an aqueous liquid fraction comprising water; and withdrawing theorganic liquid fraction from the at least one separation zone, theorganic liquid fraction comprising at least a portion of the at leastone reactant and at least a portion of the at least one product, andrecycling at least a portion of the withdrawn liquid fraction to the atleast one water removal distillation zone. In this embodiment, watergenerated in the first reaction zone is effectively removed from thefirst reaction zone.

The unique configuration of the first and second reaction zones,condensation zone, separation zone and reactant recovery distillationzone is advantageous in enabling the production of a bottoms liquidfraction (from the reactant recovery distillation zone) having highconcentrations of 2-ethylhexyl acrylate, for example, at least 50.0weight percent, and having essentially no acrylic acid. This processallows for removal of product from the reaction zone while catalystdesirably remains in the reaction system. The process of the inventiondoes not require the use of an aqueous azeotropic distillation column toseparate the desired ester product from the starting acid.

The process of this invention is particularly applicable in theproduction of esters, especially esters that contain ethylenicunsaturation or other reactive groups that can lead to unwanted sidereactions. Advantageous processes include the formation of alkylacrylates and alkyl methacrylates from alkanols of 4 to about 12 carbonatoms, and acrylic acid or methacrylic acid. A preferred aspect of thisinvention pertains to processes for making 2-ethylhexyl acrylate from2-ethylhexanol and acrylic acid.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic depiction of a process for making 2-ethylhexylacrylate from acrylic acid and 2-ethylhexanol in accordance with oneembodiment of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

This invention relates to a process for conducting equilibrium-limitedreactions. This invention pertains broadly to any equilibrium-limitedreaction process; however, the process is most useful for producingorganic equilibrium products, especially esters. The process can beconducted batchwise, but preferably is employed as a continuous processin which the reactants and any adjuvants, such as catalysts, inhibitorsand solvents, are added periodically or uninterruptedly to, and productsare removed periodically or uninterruptedly from, the reaction zones.The following discussion references the use of at least two reactantsfor the sake of convenience. It should be understood in the aspects ofthis invention where a single reactant is used for theequilibrium-limited reaction, that the description applies equally.Similarly, reference is made to a co-product for the sake ofconvenience. Equilibrium-limited reactions where no co-product is formedare also encompassed by the invention.

Typical equilibrium-limited reaction processes include esterificationand alcoholysis reactions. Esterification reactions involve theproduction of an ester by reaction of an alcohol with a carboxylic acid.A coproduct, water, is also produced. In alcoholysis(transesterification) reactions, an ester is reacted with an alcoholwith an interchange occurring.

The carboxylic acids used in the process of this invention often can berepresented by the formula R′C(O)OH, wherein R′ is ahydrocarbyl-containing group of 1 to about 8, preferably 1 to about 4,carbon atoms and may be saturated or unsaturated aliphatic orcycloaliphatic (including branched and unbranched aliphatic andcycloaliphatic which may be saturated or contain ethylenicunsaturation), aryl, alkaryl (cyclic, linear and branched alkyl),aralkyl (cyclic, linear and branched alkyl), or any of the precedingcontaining a hetero atom such as oxygen, sulfur, nitrogen andphosphorus, and R′ may be substituted with one or more heteroatom-containing substituents such as halides. In alcoholysis processes,the esters generally can be represented by the formula R′C(O)OR″ whereinR′ is as defined above and R″ is a hydrocarbyl-containing group of 4 toabout 12, preferably 4 to about 8, carbon atoms and may be saturated orunsaturated aliphatic or cycloaliphatic (including branched andunbranched aliphatic and cycloaliphatic which may be saturated orcontain ethylenic unsaturation), aryl, alkaryl (cyclic, linear andbranched alkyl), aralkyl (cyclic, linear and branched alkyl), or any ofthe preceding containing a hetero atom such as oxygen, sulfur, nitrogenor phosphorus. The alcohols can be represented by the formula R′″ OHwherein R′″ is a hydrocarbyl-containing group of 4 to about 12 carbonatoms and may be saturated or unsaturated aliphatic or cycloaliphatic,aryl, alkaryl, aralkyl, or any of the preceding containing a hetero atomsuch as oxygen, sulfur, nitrogen or phosphorus, and R′″ may besubstituted with one or more heteroatom-containing substituents such ashalides, with the proviso that in an alcoholysis reaction, R′″ is otherthan R″. The products can be represented by the formula R′C(O)OR′″.

The process of this invention can be used to simultaneously produce morethan one equilibrium product. For instance, more than one acid or estercan be used or more than one alcohol can be used to form a mixture ofesters. By the use of two reaction zones operating under differentconditions, a product stream containing one ester product may berecovered from the primary reaction zone and a higher boiling esterproduct may be recovered from the secondary reactor. Alternatively, bothproducts may be recovered from the secondary reaction zone, either bothin a vapor stream or one in the vapor stream and the other in a liquidproduct stream.

Particularly attractive uses of the process of this invention are in theproduction of acetates, acrylates, propionates and methacrylates whereinR′″ is 4 to about 12 carbons, preferably 4 to 11, more preferably 4 to8, and most preferably 5 to 8, carbon atoms. Examples of suitablealcohols include n-butanol, isobutanol, pentanol, hexanol, 2-ethylhexanol, methoxypropanol, ethoxyethanol, methoxybutanol, ethoxypropanol,ethoxybutanol, butoxyethanol, butoxyethoxyethanol, ethoxyethoxyethanol,and methoxyethoxyethanol. The carboxylic acid feed preferably contains 2to 4 carbons. Examples of preferred carboxylic acids include aceticacid, acrylic acid, propionic acid, and methacrylic acid.

The process is conventionally conducted at temperatures within the rangeof from about 0° to about 200° C., preferably in the range of from about40° to about 150° C., but below a temperature that causes unduedegradation of reactants, desired products, or any catalyst used.Temperatures that are too low result in lower reaction rates andtemperatures that are too high result in more by-products and havehigher corrosion rates. The second reaction zone temperature (togetherwith pressure) should be sufficient to crack heavies, for example,Michael-addition heavies, formed in or introduced into the secondreaction zone and to vaporize at least a portion of the ester productupon production thereof. However, the second reaction zone temperatureshould not cause undue degradation of reactants, desired products, orany catalyst used. Where a reactant contains another reactive group, forexample, unsaturation in the case of acrylic and methacrylic moieties,the temperature should be below that which causes undesirable sidereactions. To some extent, the polymerization reactions can becontrolled by the use of inhibitors, and thus the operating temperatureof the reaction zones will also be influenced by inhibitorconcentration.

The pressure under which equilibrium-limited reactions can be conductedcan also vary widely. Typically, pressures range from subatmospheric tosuperatmospheric, for example, from about 0.01 to about 100 bar, mostoften from about 0.02 to about 10 or about 15 bar absolute. As indicatedabove, the second reaction zone temperature and pressure should besufficient to crack heavies, for example, Michael-addition heavies,formed in or introduced into the second reaction zone and to vaporize atleast a portion of the ester product upon production thereof.

The reaction is conducted in the presence of a liquid medium. One ormore of the reactants, products, coproducts and side reaction productscan make up the liquid medium for the reaction. The liquid medium canoptionally comprise a solvent. The reaction menstruum in the firstreaction zone preferably is different than that in the second reactionzone. Where a solvent is used, it is preferably substantially inertunder reaction conditions.

Many equilibrium-limited reactions employ a catalyst. Catalystsappropriate for the equilibrium-limited reaction can be used in theprocesses of this invention. For esterification, catalysts are oftenacids such as sulfuric acid, sulfonic acids and acidic exchange resins,and for alcoholysis reactions, metal oxides and alkoxides such as ofalkali, alkaline earth, transition and rare earth metals, lead, bismuthand tin and the like. The catalyst is used in a catalytic amount, andthe amount of catalyst can vary widely. Homogeneous catalysts are oftenused in the range of from about 0.001 to about 10 or about 20 weightpercent of the liquid menstruum, and heterogeneous catalysts typicallycomprise from about 10 to about 60 percent of the volume of the reactionzone. Lower catalyst concentrations result in lower esterification andcracking rates and a higher purge from the first and/or second reactionzones. Higher catalyst concentrations generally make more by-productsand have higher corrosion rates. When employing sulfonic acid catalystssuch as dodecylbenzene sulfonic acid (DBSA), a small amount of watershould be present, for example, from about 0.01 weight percent or lessto about 1 weight percent or greater, preferably less than about 1weight percent, in the first and/or second reaction zones to minimizethe formation of the sulfonate ester.

Other adjuvants may be contained in the liquid reaction media, such asantioxidants, stabilizers, buffers, polymerization inhibitors and thelike. Phenothiazine (PZ) is the preferred inhibitor. Since PZ is notsoluble in water, hydroquinone (HQ) is preferably used as the inhibitorfor aqueous streams. The monomethyl ether of hydroquinone (MEHQ) is thepreferred product shipping inhibitor and is used in the product recoverydistillation column. Air or oxygen is used to enhance the effectivenessof the inhibitors. A partial pressure of oxygen of from about 0.05 toabout 1.0, preferably from about 0.1 to about 0.8, mm Hg at the columnbase is preferred for all the columns.

The process of this invention is conducted in at least one firstreaction zone and at least one second reaction zone. The at least onefirst reaction zone preferably is maintained under reaction conditionssuch that at least a portion of the at least two reactants and at leastone product are maintained in the liquid phase. Preferably, under theconditions of the first reaction zone including any azeotrope formation,the vapor-liquid equilibrium for the at least one product is such thatat least about 70, preferably at least about 80, percent of the productin the first reaction zone is in the liquid phase. Preferably, under theconditions of the first reaction zone including any azeotrope formation,the vapor-liquid equilibrium for the at least two reactants is such thatat least about 50, preferably at least about 70, percent of eachreactant in the first reaction zone is in the liquid phase.

The first reaction zone may be a single vessel or may comprise two ormore discrete vessels, one or more of which may be a stirred or agitatedtank. The first reaction zone is operated under conditions such thatessentially no cracking occurs in the first reaction zone. In oneembodiment, an overhead stream may be taken to remove co-product (forexample, water, from the esterification of alcohol with carboxylic acid)and thus drive the reaction further toward conversion to the desiredproduct. Generally, the overhead stream is subjected to rectification orother separation unit operation such as liquefaction, condensation andliquid phase separation, sorption and membrane separation, to recoverreactants for recycle to the reaction zone. In another embodiment, nooverhead stream is removed from the first reaction zone, thus permittingthe use of a plug flow reactor. Because no overhead stream need betaken, savings in equipment and energy can be achieved.

Generally, the residence time of the liquid menstruum in the firstreaction zone is sufficient to yield production in a concentrationwithin 50, typically within about 70, and sometimes at least about 90 or95, percent of the theoretical equilibrium concentration of the productin the reaction menstruum under the conditions of the reaction (forgiven reactant concentrations). Advantageously, at least about 50,preferably at least about 70, and most preferably between about 75 and95, percent of the total amount of product produced in the process isproduced in the first reaction zone.

The relative amounts of the reactants fed to the first reaction zone mayalso vary widely and will often be selected based upon economic factors.In many commercial equilibrium-limited reaction processes, the reactantsare fed in an approximately stoichiometric ratio for producing thedesired product, plus any additional amounts required to make up forlosses due to side reactions. Often, for esterification and alcoholysisreactions, the mole ratio of the alcohol to acid or ester is from about0.9:1 to about 1.1:1. Preferably, the first reaction zone is operatedsuch that an amount equivalent to at least about 50, preferably at leastabout 70, and most preferably between about 75 and 95, percent of thefresh feed of at least one of the reactants is consumed. It should beunderstood that the amount of the reactants, and their relativeconcentrations, in the first reaction zone may be different than that ofthe fresh feed due to recycling of unreacted reactants. Generally, anyrecycle of reactants is to the first reaction zone in order to enhancethe driving force to the desired product.

Liquid is withdrawn from the first reaction zone, which liquid containsproduct and reactants. At least a portion of this liquid is introducedinto a second reaction zone. While in many instances, essentially all ofthe liquid withdrawn from the first reaction zone is passed to thesecond reaction zone, the broad concept of this invention contemplates,for instance, an intervening separation step to be used to removeproduct and/or coproduct from the liquid. The separation may simply be aliquid phase separation to remove, for example, water from anesterification, a flashing or distillation unit operation, or product orcoproduct via a membrane separation or a sorption step. Also, a portionof the liquid stream may be used for other processing. Additionalreactant can be provided to the second reaction zone as a fresh feed orthrough recycle.

Like the first reaction zone, the second reaction zone may be a singlevessel or may comprise two or more discrete vessels. The conditions ofthe second reaction zone are maintained such that the product ispreferably produced in the liquid phase and then vaporized. Preferably,a boiling point reducing agent, preferably water, is present to lowerthe boiling point of the product to avoid deleterious effects of hightemperatures or expensive, high vacuum.

Preferably, under the conditions of the second reaction zone, thevapor-liquid equilibrium for the at least one product is such that lessthan about 50, preferably less than about 30, percent of the product inthe second reaction zone is in the liquid phase. In many instances,under the conditions of the second reaction zone, the vapor-liquidequilibrium for the at least two reactants results in at least onereactant being vaporized so that less than about 50, and sometimes lessthan about 30, percent of at least one of, sometimes both, the reactantsin the second reaction zone are in the liquid phase. Generally at leastabout 5, typically at least about 10, and as much as about 50, percentof the fresh reactant fed to the reaction system that is consumed in thereaction system is consumed in the second reaction zone.

Often the reactions in the second reaction zone are conducted attemperatures within the range of from about 0° to about 200° C., moretypically from about 40° to about 170° C., but below a temperature thatcauses undue degradation of the reactants, desired products, catalyst ordesirable side reactions. The second reaction zone temperature (togetherwith pressure) should be sufficient to crack heavies, for example,Michael-addition heavies, formed in or introduced into the secondreaction zone and to vaporize at least a portion of the ester productupon production thereof. Where a reactant contains another reactivegroup, for example, unsaturation in the case of acrylic and methacrylicmoieties, the temperature should also be below that which causesundesirable side reactions such as polymerization. Polymerizationinhibitors may be used to extend the desirable temperature range for thereaction. The pressure in the secondary reaction zone can also varywidely. Typically, pressures range from subatmospheric tosuperatmospheric, for example, from about 0.01 to about 100 bar, mostoften from about 0.02 to about 10 or about 15 bar, absolute.

The reaction in the second reaction zone is conducted in the presence ofa liquid comprising at least one of: (A) at least one of said reactants;(B) a product other than the substantially vaporized product, where morethan one product is intended to be formed; (C) a co-product other than asubstantially vaporized co-product, and (D) at least one other liquidcomponent, for example, a solvent. Vapor is withdrawn from the secondreaction zone and comprises (i) at least one of the reactants, (ii) theproduct, and (iii) the co-product, if any.

Where the equilibrium-limited reaction is an esterification oralcoholysis reaction, it is possible for the acid or ester to dimerizeor generate other heavies. For the esterification of acrylic acid and2-ethylhexanol to form 2-ethylhexyl acrylate, the heavies are formed bya Michael-addition reaction. The dimer or heavies product is typicallyan equilibrium product. The process of this invention facilitatescracking of the dimer and other heavies. Particularly, the secondreaction zone can be operated at sufficiently high temperatures to crackthe dimer and heavies, and the dimer and heavies may comprise asubstantial portion of the liquid menstruum, for instance, at leastabout 10 or, more typically, about 20 to about 90 or more, weightpercent of the menstruum. The heavy residues, including uncrackableheavies, polymerization inhibitors, catalyst and polymers, are purgedvia the second reaction zone tails.

In one embodiment, the process of the invention further compriseswithdrawing a bottoms liquid fraction from the second reaction zone,said bottoms liquid fraction comprising heavies, inhibitors andcatalyst, and recycling at least a portion of said withdrawn bottomsliquid fraction to at least one of the first and/or second reactionzones.

In another embodiment, the process comprises withdrawing an overheadvapor fraction from a reactant recovery distillation zone, said overheadvapor fraction comprising at least one reactant, introducing at least aportion of said withdrawn overhead vapor fraction into at least onecondensation zone to produce said at least one reactant in the liquidphase, and recycling at least a portion of at least one reactant in theliquid phase to at least one of the first and/or second reaction zones.

It is also contemplated that the process can comprise withdrawing abottoms liquid fraction from at least one product recovery distillationzone, said bottoms liquid fraction comprising heavies, and recycling atleast a portion of said withdrawn bottoms liquid fraction to at leastone of the first and/or second reaction zones.

The process of the invention can be operated such that water is producedin the first reaction zone and, under reaction conditions of said firstreaction zone, a vapor fraction comprising water, at least a portion ofsaid at least one reactant and at least a portion of at least oneproduct, is generated and said vapor fraction is withdrawn overhead fromsaid first reaction zone and introduced into at least one water removaldistillation zone.

The process of the invention can also be operated in a manner such thatat least a portion of at least one reactant is introduced into the tophalf of a water removal distillation column.

The process of this invention will be further described with respect tothe esterification of acrylic acid with 2-ethylhexanol. While this is apreferred manner to produce 2-ethylhexyl acrylate, it is not intended tolimit the broader aspects of this invention.

As a brief overview, 2-ethylhexyl acrylate is prepared by acid catalyzedesterification of acrylic acid with 2-ethylhexanol. In the processacrylic acid is esterified with a homogeneous acidic catalyst in tworeactors run in series. In the first reactor, water is removed overhead.The tails stream containing 2-ethylhexyl acrylate, unreacted reactants,and by-product is sent to a second reactor in order to increaseconversion and crack heavies. 2-Ethylhexyl acrylate, unreactedreactants, and by-products, are recovered in the overhead make of thesecond reactor. This overhead make is refined to give essentially pure2-ethylhexyl acrylate.

With reference to FIG. 1, fresh acrylic acid is fed via line 10 to afirst reactor 100. Fresh 2-ethylhexanol can be fed directly to reactor100 via line 10 and/or indirectly via line 31 and/or lines 62 and 61.The molar ratio of fresh 2-ethylhexanol to fresh acrylic acid preferablyis from about 0.9:1 to about 1.1:1. The acrylic acid and 2-ethyl hexanolsupplied to the reactor 100 are typically of standard purities. However,as a result of the processes of the present invention, higherconcentrations of typical impurities in the acrylic acid stream arebetter tolerated. For example, the acrylic acid feed to reactor 100 maycontain up to 2 or more weight percent acrylic acid dimer, an impuritycommonly present in the acrylic acid feed. Acrylic acid dimer is readilycracked by the high temperature operation of the second reactor 200 asdiscussed herein. Similarly, as a result of the unique refining scheme,it is possible to easily remove alkanol-derived impurities, which allowsthe use of a lower grade alkanol feed. The ability to use a wide rangeof acid and alcohol leads to significant economic savings.

The reaction is carried out in the presence of an acidic catalyst thatis introduced via line 10 into reactor 100. Illustrative acidiccatalysts include, for example, sulfuric acid, phosphoric acid, andresins that contain acid functional groups. Preferably, the catalyst isa long chain alkyl benzene sulfonic acid such as dodecylbenzene sulfonicacid (DBSA). DBSA catalyst and variations of it are described in U.S.Pat. No. 5,231,222, the disclosure of which is incorporated herein byreference. Relative to other catalysts, DBSA generates significantlyless impurities and heavies during the esterification of acrylic acidwith 2-ethylhexanol; hence, higher efficiencies are achieved with DBSAas a result of low impurity and heavies formation. DBSA is a homogeneouscatalyst. Nonetheless, the reaction is carried out using DBSA because,unlike conventional process employing DBSA, which would require catalystreclamation steps, in the present processes DBSA is simply cycled vialines 21 and 23 between reactors 100 and 200. The reactors 100 and 200and supply lines 10, 20, 21 and 23 are constructed of materialsresistant to corrosion by the acid catalyst.

In reactor 100 the amount of DBSA preferably is from about 0.1 to about10, more preferably from about 0.5 to about 2, weight percent of theliquid menstruum. DBSA is purged from the process during normaloperation, thus the catalyst make-up to reactor 100 or 200 can be aspure catalyst or as a solution of the catalyst with acrylic acid,2-ethylhexanol, recycle liquid or any other process stream.

It is well known in the art that chemical inhibitors are employed toinhibit the formation of polymers derived from acrylic acid and/or2-ethylhexyl acrylate. Inhibitors are provided to reactor 100 throughthe water removal distillation column 300, preferably via introductioninto the top half of that column. The inhibitors include phenothiazine(PZ), hydroquinone (HQ), and the monomethyl ether of hydroquinone(MEHQ). It is generally accepted that polymer formation occurs in areaswhere the temperature is high, such as the reactor and distillationcolumns, or in those areas where vapor condenses on cold surfaces. PZ isutilized in organic streams and HQ and/or MEHQ in water streams. Theamount of inhibitors used depends on the process. The concentration ofchemical inhibitors in reactor 100 will be from about 50 to about30,000, for example, about 10,000, ppm by weight based upon the weightof the liquid menstruum.

Besides the chemical inhibitors, oxygen is added to reactor 100 toenhance the inhibition of polymer formation. The use of oxygen is wellknown in the art. The oxygen can be added as pure oxygen, as a mixturewith an inert gas, or preferably as air. The oxygen is supplied by anair sparger provided at the bottom of the reactor (not shown).

Reactor 100 is a tank type reactor for the reaction of acrylic acid with2-ethylhexanol and is designed to allow the removal of water in order toforce the reaction equilibrium to acrylate. A conversion of about 70 toabout 85 percent is desired. A portion of the liquid in reactor 100 istaken to a reboiler (not shown) for increasing the temperature of theliquid. The reboiler can be a conventional tube in shell vessel. Thevolume turnover rate through the reboiler must assure that the contentsof the reactor are well agitated and uniformly heated. Alternatively, ajacketed reactor designed to generate the requisite heat and providedwith mechanical stirrers could be used in place of the tank reactor andreboiler.

The temperature in reactor 100 can range from about 80 to about 170° C.but it is most preferred to maintain the temperature within the range offrom about 100 to about 130° C. The return stream from the reboiler istherefore at a temperature about 5 to about 15° C. higher. The averageresidence time in reactor 100 preferably is from about 1 to about 6hours. The pressure in reactor 100 preferably is maintained at fromabout 100 to about 1000 mm Hg (0.13 to 1.33 bar) absolute. The liquid inreactor 100 preferably contains less than 1 weight percent water andpreferably is in a single phase.

The esterification reaction generates water which is removed overheadand supplied to the bottom of a water removal distillation column 300.As stated above, removing water drives the reaction toward 2-ethylhexylacrylate. The distillation column 300 may be attached to the top of thereactor. Distillation column 300 is of standard engineering design andcan use trays or packing. To accommodate any entrainment of the DBSAcatalyst, the bottom trays may need to be constructed of a metal whichcan handle highly corrosive liquid. To prevent polymerization and otherfouling reactions in the distillation column 300, conventionalinhibitors such as hydroquinone and phenothiazine are introduced vialines 31 and 32 into column 300, diluted by 2-ethylhexanol or some otherprocess liquid. Instead of feeding the alkanol starting materialdirectly into reactor 100, at least a portion of the alkanol startingmaterial may be introduced into the top section of distillation column300.

The overhead from the distillation column 300 is removed as a vapor vialine 30 and supplied to a condenser (not shown) and then to separator800, which preferably is a decanter. In the condenser, the vapor iscondensed and the resulting liquid is phase separated in separator 800,with a portion of the organic phase being returned via line 81 to thedistillation column 300 and the remainder of the organic phase beingpurged to remove any undesired low-boiling impurities and by-products,and the aqueous phase being sent via line 80 to disposal or to anotherseparation device (not shown) to recover organics from the aqueousphase.

The liquid reaction menstruum from the reactor 100 is supplied via line20 to reactor 200. Reactor 200 is a standard tank reactor equipped withan air sparger (not shown). A portion of the liquid in reactor 200 istaken to a reboiler (not shown) to increase the temperature of theliquid. The volume turnover rate through the reboiler must assure thatthe contents of the reactor are well agitated and maintained at thedesired temperature.

The operating temperature in reactor 200 is higher than reactor 100 andthe preferred range is from about 115° C. or about 135° C. to about 150°C. The return stream from the reboiler is about 5° C. to about 15° C.higher than the preferred reactor temperature. The higher temperaturenot only facilitates the conversion of the remaining 2-ethylhexanol andacrylic acid to product, but very importantly, under these conditions,enable the heavies to be cracked back to 2-ethylhexyl acrylate, acrylicacid and 2-ethylhexanol.

The operating pressure in reactor 200 preferably is lower than thepressure in reactor 100, and the preferred range is about 10 to 200 mmHg absolute (about 0.01 to 0.26 bar absolute). The residence time inreactor 200 is from about 1 to about 6 hours. Water is fed to reactor200 in order to maintain a concentration of about 1 weight percent ofthe liquid menstruum for effective catalyst operation, and to facilitatevaporization of 2-ethylhexyl acrylate. The concentration of DBSA in thesecond reactor is about 1 to about 20, preferably about 5 to about 15,for example, about 10, weight percent based upon the weight of theliquid menstruum. As with reactor 100, inhibitor is added throughoutreactor 200 to reduce polymerization.

The liquid bottom stream from reactor 200, which contains heavies andcatalyst, is recycled via lines 21 and 23 to reactor 100. The bottomstream from reactor 200 is richer in catalyst and heavies than reactor100. It should also be noted that the inhibitor concentration is alsogreater than in the first reactor, due to the cycle of the heaviesbetween reactors 100 and 200. A purge from this recycle stream can betaken via line 21. The unique reaction system of this process allows forthe recycle of heavies, catalyst and inhibitors. In conventionalprocesses the entire heavies stream containing the catalyst andinhibitors is typically discarded. Thus, the processes of this inventionenable lower catalyst and inhibitor usage and reduced inhibitor andcatalyst cost.

2-Ethylhexyl acrylate, water and unreacted acrylic acid and2-ethylhexanol are removed as a vapor from reactor 200 and supplied vialine 22 to condenser 400 and then to separator 500. Separator 500preferably is a decanter. Either fresh or recycled inhibitor isintroduced via line 41 into condenser 400. In condenser 400, the vaporis condensed and the resulting liquid is supplied via line 40 toseparator 500 where it is phase separated, with the organic phase beingsupplied via line 51 to reactant recovery distillation column 600 and atleast a portion of the aqueous phase being sent via line 50 to reactor200. Either fresh or recycled inhibitor is introduced via line 62 intodistillation column 600. The primary purpose of distillation column 600is to separate the organic liquid stream from separator 500 into a tailsfraction containing 2-ethylhexyl acrylate and heavies, and an overheadstream comprising 2-ethylhexanol and acrylic acid. The tails fraction isessentially free of 2-ethylhexanol and acrylic acid. The column designis consistent with conventional engineering practice and can use packingor trays. In this embodiment, the base temperature of distillationcolumn 600 is from about 120 to about 160° C. with a pressure of about10 to about 100 mm Hg (0.01 to 0.13 bar absolute). The overhead streamfrom distillation column 600 is recycled via line 61 back to either orboth of reactors 100 and 200.

The tails fraction from distillation column 600 is supplied via line 60to product recovery distillation column 700. Fresh inhibitor isintroduced via line 72 into distillation column 700. Column 700separates the tails fraction into an overhead stream of 2-ethylhexylacrylate and a tails stream of heavies. The tails stream is recycled vialines 70 and 73 to either or both of reactors 100 and 200. The tailsstream also contains inhibitors which, as described earlier, aredesirably recycled. The column design is consistent with conventionalengineering practice and can use packing or trays. The base temperatureof the column is about 120 to about 160° C. with a pressure of about 10to about 100 mm Hg (0.01 to 0.13 bar absolute). Alternatively, a vaporstream can be withdrawn from the bottom portion or base of column 600and condensed to form a stream of 2-ethylhexyl acrylate, thuseliminating the need for distillation column 700.

As stated above, the process of this invention can be used to make otherproducts from equilibrium-limited reactions. In the following example,2-ethylhexyl acrylate is produced by a process in accordance with thisinvention. All parts and percentages are by weight unless otherwiseindicated.

EXAMPLE 1

This process scheme of this example is shown in FIG. 1. A total of 379g/h of fresh 2-ethylhexanol, 352 g/h via line 31 and 27 g/h via lines 62and 61, and 215 g/h of fresh acrylic acid, via line 10, are fedcontinuously to a first reactor (100). The molar ratio of fresh2-ethylhexanol to fresh acrylic acid is about 1 to 1. Unreacted2-ethylhexanol and acrylic acid are recycled through line 61 to reactor100 at 172 g/h; the recycle stream contains 30 percent acrylic acid, 42percent 2-ethylhexanol, 19 percent 2-ethylhexyl acrylate, 4 percentwater, and 5 percent other compounds. Recycle inhibitor solution is fedthrough line 32 at 22 g/h. Fresh dodecylbenzene sulfonic acid catalystis fed to reactor 100 through line 10 at about 1 g/h, along with 55 g/hrecycle catalyst solution from a second reactor (200) through line 23.Reactor 100 is operated at a temperature of 125° C. and a pressure of310 mm Hg absolute. The residence time in reactor 100 is from about 4.5to about 5.5 hours. A vapor stream rich in water is removed from reactor100 and sent to a water removal column (300), which is operated at ahead pressure of 300 mm Hg absolute. The overhead vapor from column 300is condensed in a condenser (not shown) and decanted in a separator(800). The aqueous phase is removed through line 80 at a rate of 55 g/h.A portion of the organic phase is returned to column 300 via line 81 andthe remaining is removed via line 33 at a rate of 3 g/h to purgeundesired low-boiling impurities and by-products. A 786 g/h reactionliquid, containing about 1% dodecylbenzene sulfonic acid catalyst andabout 0.5% water, is discharged from reactor 100 and fed to reactor 200through line 20.

Reactor 200 is operated at a temperature of 130° C., a pressure of 40 mmHg absolute, and a residence time of 2.5 to 3.5 hours. Thedodecylbenzene sulfonic acid catalyst concentration in reactor 200 isabout 10% and the water concentration in reactor 200 is about 0.1%.Water is fed to reactor 200 through line 50 at a rate of 255 g/h. Avapor stream is removed from reactor 200 through line 22, condensed in acondenser (400) and decanted in a separator (500). The organic phasefrom separator 500 is fed to a reactant recovery column (600) via line51 at a rate of 762 g/h. The aqueous phase from separator 500 isreturned to reactor 200 via line 50. Fresh inhibitor solution is fed tocondenser 400 through line 41 at 6 g/h along with 33 g/h recycleinhibitor solution. A residue purge stream is removed from reactor 200via line 21 at a rate of 8 g/h.

Reactant recovery column 600 is operated at a head pressure of 75 mm Hgabsolute, a head temperature of about 110° C. and a base temperature ofabout 145° C. Unreacted reactants are recovered as a distillate andrecycled to reactor 100 through line 61. A liquid stream, which isessentially free of 2-ethylhexanol and acrylic acid, is recovered fromthe base of column 600 at a rate of 590 g/h and fed to a productrecovery column (700) via line 60. Fresh inhibitor solution, using fresh2-ethylhexanol as inhibitor carrier, is fed to column 600 through line62 at a rate of 27 g/h.

Product recovery column 700 is operated at a head pressure of 18 mm Hgabsolute, a head temperature of about 120° C. and a base temperature ofabout 125° C. Refined 2-ethylhexyl acrylate is recovered as a distillateat a rate of 549 g/h with a purity of 99.7 percent. A liquid stream isremoved from the bottom at a rate of 55 g/h and used as recycledinhibitor solution for column 300 and condenser 400. Fresh inhibitorsolution, using pure 2-ethylhexyl acrylate as inhibitor carrier, is fedto column 700 through line 72 at a rate of 14 g/h.

Although the invention has been illustrated by the preceding example, itis not to be construed as being limited thereby; but rather, theinvention encompasses the generic area as hereinbefore disclosed.Various modifications and embodiments can be made without departing fromthe spirit and scope thereof.

1. A process for conducting an equilibrium-limited reaction of at least one reactant to produce at least one product, comprising: a. reacting said at least one reactant in a first reaction zone maintained under reaction conditions, including temperature and pressure, sufficient to produce said at least one product and sufficient to maintain at least a portion of said at least one reactant and said at least one product in the liquid phase; b. withdrawing a liquid fraction containing said at least one reactant and said at least one product from said first reaction zone, and introducing at least a portion of said withdrawn liquid fraction into a second reaction zone maintained under reaction conditions, including temperature and pressure, sufficient to (i) produce said at least one product, (ii) crack heavies formed in or introduced into said second reaction zone and (iii) vaporize at least a portion of said at least one product upon production thereof; c. withdrawing an overhead vapor fraction from said second reaction zone, said overhead vapor fraction comprising said at least one product, and introducing at least a portion of said withdrawn overhead vapor fraction into at least one condensation zone to produce said at least one product in the liquid phase; d. withdrawing a liquid fraction from said at least one condensation zone, said liquid fraction comprising said at least one product and water, and introducing at least a portion of said withdrawn liquid fraction into at least one separation zone to provide by phase separation an organic liquid fraction comprising said at least one product and an aqueous liquid fraction comprising water; e. withdrawing said organic liquid fraction from said at least one separation zone, said organic liquid fraction comprising said at least one product, and introducing at least a portion of said withdrawn organic liquid fraction into at least one reactant recovery distillation zone to provide an overhead fraction comprising said at least one reactant and a bottoms liquid fraction comprising said at least one product; f. withdrawing said bottoms liquid fraction from said at least one reactant recovery distillation zone, said bottoms liquid fraction comprising said at least one product, and introducing at least a portion of said withdrawn bottoms liquid fraction into at least one product recovery distillation zone to provide an overhead fraction comprising said at least one product and a bottoms liquid fraction comprising heavies; and g. recovering said at least one product from said withdrawn overhead fraction.
 2. The process of claim 1 wherein in step (a) said at least one reactant comprises a crude acrylic acid stream containing acrylic acid dimer and/or other Michael-addition heavies, and an alcohol-containing feedstock comprising a crude 2-ethylhexanol stream.
 3. The process of claim 1 wherein said at least one reactant comprises a carboxylic acid-containing feedstock and/or an alcohol-containing feedstock generated from a heavy residue-containing stream from another process which employs an equilibrium-limited reaction.
 4. The process of claim 1 wherein said product is represented by the formula R′C(O)OR′″, said reactant is represented by the formula R′C(O)OH, and another said reactant is represented by the formula R′″ OH wherein R′ is a hydrocarbyl-containing group of 1 to about 8 carbon atoms and R′″ is a hydrocarbyl-containing group of 4 to about 12 carbon atoms.
 5. The process of claim 1 in which butyl acrylate and 2-ethylhexyl acrylate are simultaneously produced.
 6. The process of claim 1 in which the catalyst comprises a sulfuric acid, a sulfonic acid or an acidic exchange resin.
 7. The process of claim 1 in which less than about 1 weight percent water is present in the second reaction zone.
 8. The process of claim 1 wherein the equilibrium-limited reaction is an esterification of a carboxylic acid of 2 to 4 carbons with an alcohol of 4 to about 12 carbons.
 9. The process of claim 8 wherein the carboxylic acid comprises acrylic acid and the alcohol comprises 2-ethylhexanol.
 10. A process for conducting an equilibrium-limited reaction of at least one reactant to produce at least one product, comprising: a. reacting said at least one reactant in a first reaction zone maintained under reaction conditions, including temperature and pressure, sufficient to produce said at least one product and sufficient to maintain at least a portion of said at least one reactant and said at least one product in the liquid phase; b. withdrawing a liquid fraction containing said at least one reactant and said at least one product from said first reaction zone, and introducing at least a portion of said withdrawn liquid fraction into a second reaction zone maintained under reaction conditions, including temperature and pressure, sufficient to (i) produce said at least one product, (ii) crack heavies formed in or introduced into said second reaction zone and (iii) vaporize at least a portion of said at least one product upon production thereof; c. withdrawing an overhead vapor fraction from said second reaction zone, said overhead vapor fraction comprising said at least one product, and introducing at least a portion of said withdrawn overhead vapor fraction into at least one condensation zone to produce said at least one product in the liquid phase, and introducing at least one polymerization inhibitor into said at least one condensation zone; d. withdrawing a liquid fraction from said at least one condensation zone, said liquid fraction comprising said at least one product and water, and introducing at least a portion of said withdrawn liquid fraction into at least one separation zone to provide by phase separation an organic liquid fraction comprising said at least one product and an aqueous liquid fraction comprising water; e. withdrawing said organic liquid fraction from said at least one separation zone, said organic liquid fraction comprising said at least one product, and introducing at least a portion of said withdrawn organic liquid fraction into at least one reactant recovery distillation zone to provide an overhead fraction comprising said at least one reactant and a bottoms liquid fraction comprising said at least one product, and introducing at least one polymerization inhibitor into said at least one reactant recovery distillation zone; f. withdrawing said bottoms liquid fraction from said at least one reactant recovery distillation zone, said bottoms liquid fraction comprising said at least one product, and introducing at least a portion of said withdrawn bottoms liquid fraction into at least one product recovery distillation zone to provide an overhead fraction comprising said at least one product and a bottoms liquid fraction comprising heavies, and introducing at least one polymerization inhibitor into said at least one product recovery distillation zone; g. withdrawing from said at least one product recovery distillation zone a bottoms fraction comprising at least one polymerization inhibitor and supplying at least a portion of the withdrawn bottoms fraction to said at least one condensation zone, said at least one reactant recovery distillation zone, said at least one product recovery distillation zone and a water removal distillation zone, in an amount sufficient to minimize or eliminate polymerization of said at least one reactant and said at least one product; and h. recovering said at least one product from said withdrawn overhead fraction. 